1. Field of the Invention
The disclosed embodiments of the present invention relate to the field of circuit design, and more particularly, to transmission circuits featuring leakage prevention.
2. Description of the Prior Art
In special cases, current may flow through a leakage path from a receiving end such as a front-end receiving circuit of a High Definition Multimedia Interface (HDMI) port of a monitor or television, across a transmission line (e.g. an HDMI cable), to a transmission end such as a front-end transmission circuit of an HDMI port of a notebook or a computer. In a typical case, the receiving end is powered on before the transmission end. Even disregarding the power consumption issue, this is harmful to a circuit because electrode migration may take place, which will damage the circuit to some degree when the circuit remains in such a state for a period of time.
FIG. 1 is a circuit diagram illustrating a transmission end 10 connected to a receiving end 11. The transmission end 10 is a conventional design. When a voltage source VDDRX of the receiving end 11 is powered on but a voltage source VDDTX of the transmission end 10 is not powered on, the voltage source VDDTX will be pulled close to 0 v for a period of time. The logical value of a control signal VA is determined by a logical circuit 102, and transistors 18 and 19 are turned on in response to low input voltages VK and VQ. The non-powered voltage source VDDTX slowly pulls a control voltage VA to 0 v, and finally transistors 14 and 15 are turned on although they should remain cut-off. Normally, equivalent impedances of resistors R1, R2 and transistors 14, 15 are small, so the path from the voltage source VDDRX of the receiving end 11 to the voltage source VDDTX of the transmission end 10 has low impedance. In this situation, current leakage may occur in the receiving end 11 as a result of the transmission end 10 extracting a large current from the receiving end 11, which can be determined by Ohm's law.
In light of the above, there is a need for a novel design to solve this leakage issue.